1. Field of the Invention
This invention relates generally to batteries and particularly to lead-acid batteries and manufacture thereof. In such batteries liquid acidic electrolyte must be sealed inside the battery case to prevent leakage of the electrolyte from the battery case around a battery post or terminal extending through the case and via which electrical energy is withdrawn from the battery. While the primary thrust of the invention is towards the battery art, particularly the lead-acid battery art, the invention has applicability wherever an electrically conductive, generally metal member must be provided with a liquid-tight seal where the metal member passes through a wall, particularly a wall of an electrically insulative case, containing liquid of high or low pH or an environment otherwise corrosive to conventional sealing materials and techniques.
2. Description of the Prior Art
Mechanical fluid sealing, in the electro-chemical, corrosive environment within a battery, at lead-acid battery terminals or posts, particularly at the positive terminal post or electrode, has presented a serious problem to the battery industry for many years. Post failure structural and/or leakage at the post seal cannot be tolerated.
Over thirty years ago G. W. Vinal, in noting the problem, summarized the seals and techniques which were then used in Storage Batteries, published by John Wiley & Sons, Inc., New York, N. Y., copyright 1955. As Vinal observed, "the method of sealing the terminal posts at the point where they pass through the cover is a matter of great importance. Unless the posts are satisfactorily sealed, they are likely to work loose in the cover and cause leakage of the electrolyte."
At that time annular sleeves or flanges were typically used about the battery post to provide a seal between the post and the cover or case. If the case was hard rubber, the sleeve or flange could be force-fitted into place, but only with exercise of great care to avoid damage to the soft lead metal post. Another approach was to fit the post with an externally threaded annular sleeve which threadedly engaged a tapped hole in the hard rubber battery case. Yet another approach was to provide threads on the lead post itself and a tapped hole in the battery case or in an insert fitted into a bore through the battery case. Still another approach was to provide threads on the post with a nut thread-engaging the post and pressing down tightly on the battery case top as the nut was tightened. Sometimes the threaded battery post was further provided with a flange around the post portion inside the battery case; tightly threading the nut on to the post pulled the post flange upwards, into tight engagement with the underside of the case top. A soft gasket could be provided between the flange and the case top as additional structure to protect against electrolyte leakage at the post.
As an alternative to the various thread arrangements, a metal ring, selected from an alloy which was harder than the lead post, was burned about the exterior of the post, sealed with grease and urged against a soft gasket positioned between the metal ring and the battery cover exterior. The portion of the post inside the case was then deformed to retain the post in position with the ring squeezing the gasket against the case.
While these constructions have proved less than satisfactory, some of them remain in use even today, for want of a better post seal.
A more recent approach to the problem has utilized a relatively rigid epoxy polymer resin applied to a very well cleaned lead post over a large post area. The epoxy polymer is surrounded with a shrink-fitted, flexible, inflatable rubber tube which may also be secured to the epoxy by a suitable adhesive. The lower portion of the rubber tube forms a bag secured using suitable adhesive to an annular flange of the battery case surrounding and extending inwardly from the battery case post orifice. There may also be provided a rib or dam inside the rubber tube so that the epoxy polymer resin may be applied to the post with the dam serving to limit downward flow of epoxy polymer resin along the post. When the rib or dam is used, the tube is not shrink fitted to the post but is merely positioned about the post prior to pouring the epoxy, in order to leave a void for the epoxy to occupy; in this case the flexible, inflatable tube acts as a mold for the epoxy polymer resin. In either case, the flexible, inflatable tube is physically separated from the battery case along most of the axial length of the flexible, inflatable tube by the epoxy polymer resin; the seal intended to prevent escape of electrolyte is provided by adhesive between the flexible, inflatable tube and the case. This flexible, inflatable tube approach is generally disclosed the Bell System Technical Journal, volume 49, number 7, page 1405, copyright 1970 and in U.S. Pat. No. 3,652,340. While this approach has had some acceptance, the approach has not proven sufficiently reliable, particularly in small batteries where the area of rigid epoxy polymer resin-post contact is necessarily relatively small. The poor reliability of such epoxy seals has apparently been due at least in part to poor bonding at the rigid epoxy polymer resinpost interface, between the rigid epoxy polymer resin and the lead or lead alloy of which the post is fabricated.
A variation on the old threaded post approach disclosed by Vinal is presented in U.S. Pat. No. 4,245,014 in which the post itself is not initially threaded; rather a self-threading or self-tapping, sealant-containing member or nut is forceably threaded onto the post, cutting threads into the lead or lead alloy post and releasing sealant while engaging the post. This is combined with a case cover having a depression formed about the post orifice, which depression is filled with epoxy after the sealant-containing member is threaded into engagement with the post. The post is also equipped with a sholder for abutting the underside of the case cover depression; a rubber O-ring is positioned around the post, squeezed between the post shoulder and the underside of the case cover depression.
Yet another approach to the problem is disclosed in U.S. Pat. No. 4,522,899 where a synthetic plastic, preferably a modified polyolefin having elastomeric characteristics, is initially injection molded in an annular, toroidal O-ring-like configuration about a tapered post. The U.S. Pat. No. 4,522,899 approach relies on shrinkage of the injection molded synthetic polyolefin to provide a seal at the O-ring--post interface. Once the initial O-ring injection molding operation is complete, a second injection molding operation is performed, surrounding the annular, toroidal O-ring configured elastomeric polyolefin with a pot or cylindrical mass of a less elastic material, preferably the same plastic material as the battery case, specifically polyethylene or polypropylene. The pot or cylindrical mass of material is then heat or untrasonically bonded to the battery case.
While the approach of the U.S. Pat. No. 4,522,899 has superficial attraction, the approach has not proved to be a satisfactory solution to the post sealing problem. The injection molding process by which the annular, toroidal, synthetic plastic O-ring is fabricated about the post does not produce a good seal at the post--synthetic plastic O-ring interface. Indeed, injection molding, being a fabrication process as opposed to a bonding process, has not yielded a satisfactory bond and associated seal between the molded-in-place synthetic plastic O-ring and the metal post. Moreover, the elastomeric materials specified, such as polypropylene, tend to pull away from the battery post as these materials solidify during post-molding cure.
Lead and lead alloys are difficult to bond to other non-lead based metals and to non-metallic materials. Lead oxidizes freely and, hence, always has a small film of lead oxide present, which inhibits bonding. Material selection for post seals is limited because lead corrodes in base environments. The post sealing problem is particulary acute at the positive terminal post in a lead-acid battery because porous lead peroxide forms at that terminal. The sulfuric acid electrolyte easily creeps through the porous lead peroxide. (Surface tension of the acid electrolyte causes the acid to creep across and along all kinds and configurations of surfaces.) The sulfuric acid eventually finds its way through the epoxy polymer resin--lead or lead alloy post interface provided in the newer post seal constructions or creeps around the gaskets and the like used in the older techniques. This results in a leak and/or crevice corrosion, leading to degraded battery performance and battery failure. Leakage about the post with currently used seal techniques may also result from damage to the post seal during battery handling, from gassing in the event the battery is overcharged or from capillary rise of electrolyte along the post.
Batteries are increasingly being used in stand-by applications, to supply power in emergencies for computer systems, telephone switching equipment and the like, in the event of a power dropout or failure of the electric utility. Such stand-by batteries represent a significant capital investment and must have a useful life of many, as opposed to several, years. The required extended life aggravates the problem of providing an adequate, reliable and long lasting post seal because of the longer period over which the corrosive environment in the battery has access to the post seal construction.